How long do you have? Which workouts are you already doing? Kettlebells are definitely not a bad idea as you'll be manipulating a lot of objects.
I would do something like this:
Strength: A basic squat and deadlift variation that suits you and your body type. Some lunges or walking lunges with weights or a weight west. A horizontal and a vertical pulling variation either with BW or kettebells. Some presses either with BW or kettlebells. A variation of a Turkish getup that you can handle. Then some carries, like walking with a hexbar/trapbar with some weight, or if you have a dummy or heavy bag of sand you can practice firemans carry variations. You can work with different sizes if you have them, or better yet a friend if possible. Some walking prowler pulling with a rope, walking backwards and forwards both, could be a good exercise as well. Add a few accessory/isolation exercises to target some weak points, pain or to strengthen joints. Your shoulders, back and knees will thank you if you take care of them, so practice good technique, especially if you're going to be a firefighter.
Cardio: LISS runs (or cycling, or jumprope, or whichever, although running will have the most carryover if your joints can handle it) 2-3 times a week. Keep HR 120-150, depending on your fitness from 30-60 minutes. HIT or anaerobic work 2 times a week for short to medium durations, like upwards hill sprints (walk down), some running or climbing stairs, and some rowing on the rowing machine (this one is great) at a high pace. You could do these at the end of your strength sessions if they are not too long and you are pressed for time, as anaerobic work synergizes somewhat with strength work.
Work your way up, build up your fitness gradually, control the resistance and be consistent.
Maybe others can chime in!
PS: I grew up near a firestation, my father was a firefighter for 32 years in the capital over here and I'm a physio.
